The present invention relates to a brushless electric motor which has a rotor magnet, a coil apparatus for electromagnetically driving the rotor magnet and a rotor shaft for transmitting a rotary movement to an element or an apparatus which can be moved. The rotor shaft being firmly connected to the rotor magnet and runs through a rotation axis of the rotor magnet. There is at least one electromagnetic transducer element for detection of the magnetic field of the rotor magnet.
Electric motors such as these are available in various sizes and versions, are used in particular in handheld appliances, in particular medical and especially dental handheld appliances. The motors are used as drive means, for example, for cutting or grinding appliances, in particular for drills.
In order to drive a brushless electric motor such as this correctly and as efficiently as possible, it is necessary, in particular, to drive the coil apparatus, in general a stator winding, as a function of the respective instantaneous position of the rotor magnet. The more accurately the so-called commutation (the offset between the flux of the stator field with respect to the rotor field produced by the rotor magnet) is set, the better is the operating behavior of the motor. The efficiency is improved, less heat is developed by the motor, and higher torques and rotation speeds can be achieved, with high rotation speeds being of particularly high importance with respect to the use of a motor such as this in a medical, in particular a dental, appliance, such as a dental drill.
It has therefore been proposed in the prior art to provide an electric motor, which, in addition to the rotor magnet, has a further, preferably small, control magnet which, in particular is connected via the rotor shaft and has a fixed positional relationship with the rotor magnet. Thus, it is possible by detection of the position of the control magnet, with the position essentially being an angular position during rotation about the rotor shaft, to draw conclusions about the position of the rotor magnet, so that it is possible to drive the brushless electric motor via this “circuitous route”.
This option was proposed in order that the electromagnetic transducer elements which are intended to detect the position of the control magnet (and hence indirectly of the rotor magnet) can be arranged at a distance from the coil apparatus and from the stator winding in order that they are not influenced by them.
However, brushless electric motors such as these have the disadvantage that the position of the control magnet must be matched extremely exactly to the position of the rotor magnet. Furthermore, it is necessary to prevent rotation of the control magnet with respect to the rotor magnet before and during operation of the electric motor or during maintenance work, etc, since this would lead to incorrect information with respect to the position of the rotor magnet and of the corresponding rotating field, and would result in the electric motor and the coil apparatus being driven inefficiently.
Complicated adjustment effort must therefore be accepted in order to set up the control magnet in an optimum manner. Furthermore, there is always a risk during operation of the electric motor of the control magnet and rotor magnet rotating with respect to one another, which would once again corrupt the sensor values from the electromagnetic transducer elements.
Furthermore, the production of a control magnet such as this is highly costly, and the control magnets are relatively sensitive, which is further exacerbated by the fact that the control magnets should be kept as small as possible owing to the miniaturization requirements. For this reason they are frequently brittle and can break not only during installation and adjustment but also during operation, thus resulting in the risk of fragments which damage or destroy other parts of the motor.
German Patent Application DE 100 33 577 A1 has disclosed a brushless motor in which electromagnetic transducer elements are arranged adjacent to the coils of a coil apparatus, outside its winding parts and between the coils, even in the external circumference around the rotor magnet.
Since the electromagnetic transducer elements are, however, in this case arranged in the immediate vicinity of the coils and between them, they are heavily influenced by the magnetic field of the coils. Thus, the arrangement of the electromagnetic transducer elements between the coils must be selected very carefully and must be fixed exactly, in order to keep the influence of the coil apparatus as low as possible.